Polymers having a functional group at each of a plurality of molecular termini are known to crosslink by themselves or when combined with an appropriate curing agent and thereby give cured products excellent in heat resistance, durability, etc. Alkenyl-, hydroxyl- or crosslinking silyl-terminated polymers are typical examples. Alkenyl-terminated polymers are cured by crosslinking with a hydrosilyl group-containing compound as a curing agent or when subjected to photochemical reaction. Hydroxyl-terminated polymers, when reacted with a polyisocyanate, are cured under formation of urethane crosslinks. Crosslinking silyl-terminated polymers absorb moisture and give cured products in the presence of an appropriate condensation catalyst.
As examples of the main chain skeleton of such alkenyl-, hydroxyl- or crosslinking silyl-terminated polymers, there may be mentioned polyether type polymers such as polyethylene oxide, polypropylene oxide and polytetramethylene oxide; hydrocarbon type polymers such as polybutadiene, polyisoprene, polychloroprene, polyisobutylene, and hydrogenation products derived therefrom; polyester type polymers such as polyethylene terephthalate, polybutylene terephthalate and polycaprolactone and so forth. They are used in various fields of application according to the main chain skeleton and mode of crosslinking thereof.
Unlike those polymers obtainable by ionic polymerization or polycondensation, those functional group-terminated vinyl polymers which are obtainable by radical polymerization have scarcely been put to practical use. Among the vinyl polymers, (meth)acrylic polymers have those characteristics, such as high weathering resistance and transparency, among others, which cannot be attainable with the polyether type polymers, hydrocarbon type polymers and polyester type polymers mentioned above. Among them, those having an alkenyl or crosslinking silyl group on their side chain(s) are utilized in highly weathering resistant coatings. In the case of acrylic polymers, on the other hand, it is not easy to control the polymerization thereof because of side reactions involved therein; it is very difficult, for instance, to introduce a functional group into a terminus thereof.
If vinyl polymers having a crosslinking functional group, such as an alkenyl group, at a molecular chain terminus can be obtained in a simple and easy manner, cured products superior in cured product physical properties as compared with those having a crosslinking functional group on a side chain or chains will possibly be obtained. Thus, a number of researchers have so far searched for a method of producing them. It is not easy, however, to produce them industrially.
Japanese Kokai Publication Hei-05-255415 discloses a method of synthesizing (meth) acrylic polymers having an alkenyl group at both termini which uses an alkenyl-containing disulfide as a chain transfer agent. Japanese Kokai Publication Hei-05-262808 discloses a method of synthesizing (meth) acrylic polymers having an alkenyl group at both termini which uses a hydroxyl-containing disulfide as a chain transfer agent to thereby synthesize (meth)acrylic polymers having a hydroxyl group at both termini and then further utilizes the reactivity of the hydroxyl group. By these methods, however, it is not easy to realize the alkenyl group introduction into both termini with certainty. Further, for attaining terminal functional group introduction with certainty, it is necessary to use a chain transfer agent in large amounts. This is a problem from the production process viewpoint.
Accordingly, it is an object of the present invention to facilitate the production of vinyl polymers having a functional group at a molecular chain terminus by realizing the introduction of the functional group into a molecular chain with certainty by using a compound which is relatively readily available.